1. Field of the Invention
This invention relates to curing of polyepoxide resins and the curing of mixtures of polyepoxide resins and polyacrylate esters to produce hard, insoluble, infusible films, castings and adhesives.
2. Description of the Prior Art
Polyglycidyl ethers, particularly those prepared from a dihydric phenol such as Bisphenol A, i.e., 2,2-bis(4-hydroxyphenyl) propane, and an epihalohydrin such as epichlorohydrin, also referred to as epoxy resins, epoxide resins, polyepoxide resins or polyepoxides, have become increasingly important commercially in recent years. When cured, these thermosetting resins form insoluble, infusible films, pottings, castings, adhesives, and the like, and are markedly superior in their physical, chemical, and electrical properties to many other cured thermosetting resins. They exhibit low shrinkage during curing. The combination of hardness and toughness exhibited by the cured resins, their high adhesive strength, resistance to degradation by solvents and other chemicals and their electrical properties, such as dielectric constant and resistivity, are outstanding. At the same time, these properties can be varied within wide limits depending on the end use intended for the resin. Of the wide variety of curing agents(hardeners) and homopolymerization catalysts which have been used to cure polyepoxide resins, no one is suitable for all applications, and many have serious drawbacks no matter what the application.
Many materials are capable of curing epoxy resins at elevated temperatures, but in general, only two broad classes are sufficiently reactive to cure glycidyl ether type epoxy resins, such as the epoxynovolac resins and Bisphenol A diglycidyl ether at room temperature and below. These are mercaptans, primary and secondary polyamines and substituted amines such as the polyamidopolyamines. The cure rate of mercaptans is much faster than any other type at room temperature and below. It would be advantageous, under these conditions, to employ co-curing agents with the polymercaptan curing (hardening) agents and catalysts so that the cost would be reduced and physical properties could be varied. However, use of co-curing agents usually results in a sacrifice of gel time. That is, the formulation may lose its mercaptan cure character, becoming sluggish in gelling and development of the physical properties. In other words, the slower curing polyamines used as co-curing agents dilute the effectiveness of the catalyzed mercaptan.
Blends of primary and secondary amines and mercaptans are widely employed in the art of formulating or compounding to provide a specific balance of properties. The amine contributes improved adhesion, water resistance, rigidity and heat resistance to the cured composition, while the mercaptan contributes its unusually fast reaction or cure rate. However, the reaction rate obtained from blends of mercaptans and amines is approximately an average of the rates of the individual species and the proportion of each present. Examples of amines employed commercially as co-curing agents with mercaptans are triethylene tetramine, N-aminoethyl piperazine, commercial amido substituted polyamines such as the Genamid.RTM. products and commercial polyamido substituted polyamines such as the Versamid.RTM. products, both of the Henkel Corp.
U.S. Pat. No. 2,919,255--Hart--Dec. 29, 1959, describes blends of a conventional reactive polyamidopolyamine and a high molecular weight dimercaptopolysulfide polymer for curing an allyl substituted epoxy resin, employed as a side-seam cement for food containers. However, so little mercaptan was present that the composition had long pot life and slow cure at ambient temperatures and was generally cured at 180.degree. C. This polymer had insufficient mercaptan active hydrogen to cure a significant portion of the epoxy and was used to gain flexibility, extensibility and related properties. No amino acids were used in the preparation of the polyamides.
Further, British Pat. No. 885,880 and corresponding U.S. Pat. No. 3,036,975--Taub--May 29, 1962--disclose reaction products of .epsilon.-caprolactam and polyamines such as tetraethylene pentamine to form polyamidopolyamines. The briefest mass gel time occurred in thirty minutes at room temperature. A post cure of four hours at 120.degree. C. was used for curing. Where Versamid.RTM. 125 (reaction product of dimerized linoleic acid and diethylene triamine) was used for comparison purposes, the cure schedule used was two hours at room temperature followed by 24 hours at 150.degree. C. Co-curing with other hardeners such as polymercaptans is not described.
British Pat. No. 1,065,363--Tilley--Apr. 12, 1967 describes hardening agents which are reaction products of amino acids or their lactams, polyamines and dicarboxylic acids. While cure is effective at room temperature, the rate is relatively slow, i.e., 24 hours. Further, gel times (mass) even at 50.degree. C., range from 22 to 97 minutes. Co-curing with other hardeners such as polymercaptans is not described.